Magnetic Imaging

Summary:

Our goal is to develop a real-time magnetic domain imaging system and apply it to technologically important systems, including data-storage and permanent magnets with increased energy products, in order to test and publicize its unique capabilities. In this project we will develop methods to visualize magnetization reversal processes to explain the mechanism and ramifications of exchange interaction on the remagnetization processes in composite magnetic materials comprised of species with dissimilar bulk magnetic characteristics.

Description:

Advanced magnetic devices and storage media will rely on ultra thin ferromagnetic films; since such films are quasi two-dimensional magnets, they can have strong perpendicular magnetic anisotropy (PMA). Optimization of future materials, including improved yields, requires an ability to measure film interactions. The advanced magneto-optical indicator film (MOIF) technique is a particularly appropriate tool to obtain such information.

This technique uses a Bi-substituted yttrium iron garnet film with in-plane anisotropy, placed on top of a specimen to be analyzed. This enables the observation of the magnetic stray fields above the specimen through the magneto-optical double Faraday effect of the garnet film. This optical image of the stray magnetic fields is observed by polarized light optical microscopy. The digital difference technique is used for quantitative analysis of the magneto-optical images.

Advantages of the MOIF method:

Real-time visualization of the magnetization dynamics.

Any magnetic material can be investigated and only magnetic information is observed.

Quantitative,

Observations over a wide-temperature range

It is simple in operation and inexpensive to construct.

Major Accomplishments:

Developing new magnetic materials for magnetic storage devices requires a deep understanding of the remagnetization processes. To help the industry in such an important task, NIST is using the advanced Magneto-Optical Imaging technique to study magnetization reversal processes in magnetic materials with perpendicular magnetic anisotropy, such as [Co(0.4 nm)/Pt(1m)]4 multilayers. This study by magneto-optical Kerr effect microscopy revealed details of unusual magnetization reversal in the evolution of the domain structures in different field regimes.

Using magneto-optical Kerr microscopy and magnetic force microscopy an unexpected phenomena during magnetization reversal in ultrathin Co films and Co/Pt multilayers with perpendicular anisotropy was discovered. We have for the first time observed asymmetrical nucleation centers where the reversal begins for one direction of the field only and is characterized by an acute asymmetry of domain wall mobility. We have also observed magnetic domains with a continuously varying average magnetization, which can be explained in terms of the coexistence of three magnetic phases: up, down, and striped.

During investigation of a Pt(10nm)/Co(0.3-0.6 nm wedge)/Pt(3nm) trilayer, simultaneous domain imaging and transport measurements showed that an antisymmetric magnetoresistance (MR) found in this material is due to the appearance of domain walls that run perpendicular to both the magnetization and the current, a geometry existing only in materials with perpendicular magnetic anisotropy. As a result, the extraordinary Hall effect (EHE) gives rise to circulating currents in the vicinity of the domain walls that contributes to the MR. We have consequently first demonstrated and explained a new form of MR, which is antisymmetric in H, in multilayers with PMA.

The above results are important to the recording industry developing magnetic materials for practical applications in perpendicular magnetic recording because it shows why the reversal kinetics between different regions may vary, and it also shows there will be unexpected electrical voltages created which will vary with magnetic field direction and magnitude. Such information is obviously crucial to the device designers.